Best:
Alaska - unique design, and not too difficult to draw
California - I'm generally against words on flags, but this one works despite it. The bear also looks awesome.
Colorado - simple, but it looks good enough
Hawaii - not terribly original, but it's reasonably "flag-like"
New Mexico - simple and elegant, and very distinctive
South Carolina - don't know about you, but I find it very pleasing.
Maryland - overly complicated, perhaps, but it's unique.
Tennessee, Mississippi, Texas - not particularly outstanding, but at least it's not a coat of arms.
Worst:
South Dakota - includes the "state nickname", which is bad even by US standards.
Montana, Kansas, Wisconsin - many state flags have the state names in big letters on them, but these are the worst.
Washington - a portrait for a flag? Come on...